lim(x→0) (arctanx - sinx)/x³,洛必达法则
= lim(x→0) [1/(1 + x²) - cosx]/(3x²)
= (1/3)lim(x→0) (1 - cosx - x²cosx)/(x² + x⁴),洛必达法则
= (1/3)(1/2)lim(x→0) (sinx - 2xcosx + x²sinx)/(x + 2x³),洛必达法则
= (1/6)lim(x→0) (- cosx + 4xsinx + x²cosx)/(1 + 6x²)
= (1/6)(- 1 + 0)/(1 + 0)
= - 1/6